1. Field of the Invention
This invention relates to telephone ringing circuits and, more particularly, to electronic isolating circuits for selectively operating and extending the range of the ringers on such lines.
2. Description of the Prior Art
When more than one party is connected to a telephone line, a ringer connected between one of the conductors and ground can be provided for each party. By superimposing positive or negative direct current voltages on the ringing signal and connecting this composite ringing signal between either one of the two telephone conductors and ground, four-party fully selective ringing can be provided. Gas tubes have been used to disconnect or isolate the ringer circuit from the telephone line in the absence of ringing signals. A properly poled superimposed direct current voltage, however, breaks down the gas tube to connect the selected ringer to the appropriate conductor. In doing so, however, the high voltage drop across the gas tube limits the ringing range and the number of ringers which can be connected for a single party.
Solid state switching circuits have been applied as ringer isolators to reduce the voltage drop across the connecting device. One such circuit is disclosed in H. W. Ott U.S. Pat. No. 3,916,111, granted Oct. 28, 1975, and assigned to applicant's assignee. While such circuits have been adequate to detect superimposed ringing signals under ideal conditions, they have not always worked properly when exposed to large variations in these ringing signals. These variations can be caused by induced currents from power lines or result from the characteristics (especially the crest factor) of the ringing generator itself. There is therefore a need for improved ringer isolator circuits to extend the reliable ringing range in single and multiparty telephone service and, moreover, to provide such capability at a reduced cost in comparison to what has heretofore been possible.